One of the most frequently used pieces of weight training apparatus is the dumbbell, which comprises a short cylindrical bar with weighted discs mounted at each end. Dumbbells are most often used in pairs but may also be used singly. A dumbbell typically comprises a cylindrical steel bar, which may be chrome-plated and provided with a knurled grip portion, and weighted discs which may be fixed to or integral with the ends of the bar, or may be removable to allow a user to vary the weight of the dumbbell. The majority of dumbbells provided with removable discs use steel to form the discs, though discs formed of concrete-filled plastic shell are also available.
Dumbbells may be used in a wide variety of exercises, in the majority of which the user grips the bar with one hand. However, a number of exercises require the user to use both hands and to, for example, hold the dumbbell by one of the discs. Such exercises are generally easily accomplished when using relatively light weights. However, when heavier weights are used, the physical size of the weighted discs necessary to provide the required weight, and also the location of the discs, may restrict the movement of the user and prevent the range of movement necessary for the proper execution of the exercise. Also, having the centre of gravity of the dumbbell located in line with the bar tends to make the control and balance of the heavily-weighted dumbbell more difficult.
The removable discs of "adjustable" dumbells are normally held on the bar using a locking collar. If a collar is improperly fitted, or loosens during use, there is a danger that the discs will fall from the bar. Also, when not in use, dumbbells are normally supported by points on the circumference of the weighted discs, such that when positioned on an inclined surface, a dumbbell is liable to roll to a position where it may become a hazard, this being particularly dangerous in an area where people are likely to be exercising with heavy weights.